The present invention relates to emitters and receivers of acoustic waves in which a transducer element of nondevelopable form serves for converting an electric AC voltage into vibrations or vice versa. It concerns more particularly loudspeakers and microphones in which the dome-shaped membrane is formed by a self-supporting structure made from a polymer material. The concave and convex faces of this structure are covered with capacitor-forming electrodes. The transducer effect used in these structures appears over the whole extent of the electrosensitive zones situated between the electrodes, which allows entirely active domes to be formed. The polymer materials used for manufacturing the active domes are in the form of homogeneous or dimorphous films whose thicknesses are generally between some tens and some hundreds of microns. In this case, the final shape may be obtained by thermoforming or electroforming. Self-supporting structures with very thin walls may also be obtained by molding or by coating.
Whatever the manufacturing technique used, the dome obtained has good mechanical strength because of the self-supporting properties which distinguish it from a flat film of comparable thickness. Nevertheless, by exerting a thrust in the center of the convex face of a dome, a mechanically stable stove-in portion may be created which completely changes the nature of the electro-acoustic properties. This buckling phenomenon is reversible, but to find again the initial shape it is necessary to exert a thrust in the opposite direction to that which caused the staving-in. In practice, the user does not have access to the convex face of a dome-shaped membrane, which involves delicate dismantling of the transducer when its membrane has been accidentally staved in. To palliate this disadvantage, the convex radiating face of an active dome may be protected by a grid, but this means is inoperative when the staving-in results from an overpressure. Furthermore, staving-in may sometimes cause breaks such that the dome cannot assume again completely its original shape. In addition to accidental staving-in which may occur during use of an active-dome electro-acoustic transducer, it should be pointed out that parasitic vibratory modes may appear and give rise to irregular deformations by stationary waves. Furthermore, the vibration of an active dome tends to be amplified by resonance in a narrow range of the acoustic spectrum, which is prejudicial to a good sound reproduction. The control of the frequency response characteristic of a polymer active dome is based on damping of its natural resonance and of those which may be caused to act by acoustic coupling. However, the modest efficiency of piezoelectric polymer transducers does not allow a purely electric damping of the resonances to be contemplated which is both simple to put into practice and sufficiently efficient.